1. Field of the Invention
The present invention relates to a ceramic dynamic-pressure bearing, a motor having a bearing, a hard disk drive, a polygon scanner and method for manufacturing a ceramic dynamic-pressure bearing.
2. Description of the Related Art
Conventionally, a ball bearing has often been used as a bearing for the shaft of a motor serving as a drive unit of electric equipment. High-speed rotation of a motor has been rapidly implemented in precision equipment, such as peripheral equipment of a computer. In this connection, in order to obtain excellent bearing performance with low rotation-speed fluctuation and reduced noise and vibration, or in order to elongate bearing service life, a dynamic-pressure bearing, which uses fluid, such as air, as a medium, has been employed. The dynamic-pressure bearing operates in the following manner: when, for example, a spindle and a bearing member disposed to surround the spindle undergo relative rotation about an axis, the axis of rotation is supported by the action of fluid dynamic-pressure generated in the gap formed between the outer circumferential surface of the spindle and the inner circumferential surface of the bearing member. Also, a certain other bearing is configured such that the thrust face of a spindle or that of a bearing member is supported by action of dynamic pressure.
3. Problems to be Solved by the Invention
When a dynamic-pressure bearing as described above is in a high-speed rotation state, in which generated dynamic-pressure is sufficiently high, two members which face each other with a dynamic-pressure gap present therebetween do not come into contact with each other. However, at the time of starting or stopping, when rotational speed is low, sufficiently high dynamic pressure is not generated; thus, the two members come into contact with each other. Component members of such a dynamic-pressure bearing have generally been formed of a metal, such as stainless steel, and in some cases have been further coated with resin or like material. However, the two metallic members may cause a problem of wear or seize-up caused by mutual contact thereof at the time of starting or stopping.
In order to prevent the above-described seize-up of a dynamic-pressure bearing at the time of starting or stopping, either or both of the spindle and the bearing have been formed of a ceramic, such as alumina, which is not prone to seize-up and exhibits excellent wear resistance.
However, even when a dynamic-pressure component is formed of a ceramic, a problem may arise such that vibration occurs during rotation of a spindle, which hinders smooth rotation of the spindle. Further, when one of the spindle and the bearing is formed of a metal, seize-up may occur. In order to prevent this problem, a ceramic dynamic-pressure bearing has been proposed, configured such that the two members which face each other with a dynamic-pressure gap present therebetween are formed of ceramic. However, sufficiently smooth rotation has still not been realized.
Furthermore, in order to increase dynamic pressure to be generated, a dynamic-pressure bearing has employed dynamic-pressure grooves formed on a dynamic-pressure gap definition surface. In the case of a ceramic dynamic-pressure bearing, the dynamic-pressure grooves have been engraved by sandblasting.
However, a sandblasting process for engraving dynamic-pressure grooves involves masking with a wear-resistant material, which is troublesome. Since blasting of abrasive grains tends to be uneven, variations in groove depth tend to arise. Additionally, since the number of workpieces which can simultaneously undergo a groove formation process is limited, productivity is poor. Further, the inner surface, particularly a bottom surface, of a dynamic-pressure groove thus formed tends to become rough, and a meeting portion where a sidewall surface and a bottom surface meet tends to assume an irregular sharp shape, thereby preventing smooth generation of dynamic pressure and potentially causing center runout or vibration.
It is therefore a first object of the present invention to provide a ceramic dynamic-pressure bearing which can realize smooth rotation. A second object of the invention is to provide a ceramic dynamic-pressure bearing having dynamic-pressure grooves capable of smoothly generating dynamic pressure and allowing for excellent productivity, a method for manufacturing the dynamic-pressure bearing, and a motor having a bearing, a hard disk drive, and a polygon scanner which employ the dynamic-pressure bearing.
The above-described first object of the invention, has been achieved by providing:
(1) A ceramic dynamic-pressure bearing in which, when either a spindle or a bearing serving as a rotation body rotates relative to the other, the rotary surfaces (i.e., radial dynamic-pressure gap definition surfaces) of the spindle and the bearing come into a non-contacting state, at least the rotary surface of the spindle and/or the bearing is formed of a ceramic, and the surface porosity of the rotary surface is 10 to 60%.
When a ceramic material is used for a dynamic-pressure bearing, the surface state of the rotary surface of the ceramic component serving as a spindle or a bearing becomes important. That is, in general, fine pores are present on the surface of a ceramic component that has been subjected to polishing, due to dropping off of particles during the course of polishing; and the number, size, and distribution of such pores are considered to greatly effect the state of rotation of the dynamic-pressure bearing.
Specifically, when pores of large diameter are present on a rotary surface of the ceramic component, turbulence is generated in the fluid layer present between the spindle and the bearing upon rotation of, for example, the spindle, so that vibration of the spindle occurs. By contrast, when the number of pores present on a rotary surface of the ceramic component is excessively small, or when a large number of pores of small diameter are present on the rotary surface, adhesion easily occurs between the rotary surfaces of the spindle and the bearing, so that seize-up may occur when the spindle or the bearing is formed of a metal.
In the above first aspect of the present invention, because the surface porosity of the rotary surface formed of a ceramic is set to 10 to 60%, the size and number of pores become proper, so that occurrence of vibration or seize-up can be prevented. Further, in a dynamic-pressure bearing having a structure such that a rotation body (i.e., bearing member) is sandwiched between thrust plates, the occurrence of linking can be avoided.
The term xe2x80x9csurface porosityxe2x80x9d means the ratio of the total cross-sectional area of pores (the total area of cross sections of pores taken along a rotary surface) to the area of the rotary surface. When known dynamic-pressure grooves are formed on the rotary surface, the area of the dynamic-pressure grooves is omitted for calculation of the surface porosity. That is, in such case, the surface porosity is represented by (the total cross-sectional area of pores present on the rotary surface excluding a region where the dynamic-pressure grooves are formed)/(the area of the rotary surface excluding the region where the dynamic-pressure grooves are formed). This definition for surface porosity will be applied to the following description in the present specification.
(2) The ceramic dynamic-pressure bearing as described in (1) above, wherein the surface porosity of the rotary surface is 20 to 50%.
This embodiment of the invention limits the surface porosity to a more desirable range within the range defined in (1). Within the limited range, the above-described variation, seize-up, and linking can be prevented more effectively.
The above-described second object of the present invention has been achieved by providing a ceramic dynamic-pressure bearing comprising a dynamic-pressure gap formed between a first member and a second member, which undergo relative rotation about a predetermined axis of rotation, and the relative rotation of the first member and the second member generates fluid dynamic-pressure in the dynamic-pressure gap,
wherein at least a portion of at least either the first member or the second member which includes a surface (hereinafter referred to as a xe2x80x9cdynamic-pressure gap definition surfacexe2x80x9d) facing the dynamic-pressure gap is formed of electrically conductive ceramic, and dynamic-pressure grooves are formed on the ceramic dynamic-pressure gap definition surface such that, on a cross section of a dynamic-pressure groove taken perpendicular to the longitudinal direction of the groove, a curvature portion having a radius of 3-7 xcexcm is formed at a meeting position where a groove sidewall surface and a groove bottom surface meet.
According to the above-described configuration, on a cross section of a groove taken perpendicular to the longitudinal direction of the groove, a curvature portion having a radius of 3-7 xcexcm is formed at a meeting position where a groove sidewall surface and a groove bottom surface meet, whereby dynamic pressure can be generated more smoothly, and the occurrence of center runout or vibration becomes unlikely. A curvature portion having a radius that is less than 3 xcexcm fails to yield sufficient effect, and a curvature portion having a radius in excess of 7 xcexcm is difficult to form for an ordinary width range of a dynamic-pressure groove.
The above-described ceramic dynamic-pressure bearing can be manufactured by electrolytically etching dynamic-pressure grooves on a dynamic-pressure gap definition surface formed of an electrically conductive ceramic. Electrolytic etching renders electrolytic concentration unlikely to occur at a meeting position where a groove sidewall surface and a groove bottom surface meet, and is therefore suitable for imparting a curved surface. Electrolytic etching can finish the inner surface of a dynamic-pressure groove more smoothly as compared with sandblasting, thereby contributing to smooth generation of dynamic pressure. Specifically, the bottom surface of a dynamic-pressure groove can be a smooth surface having an average roughness along the centerline of about not greater than 1.5 xcexcm. The lower the average roughness along the centerline of the inner surface of a dynamic-pressure groove, the better. However, because of a trade-off between cost and reduction in roughness, setting a lower limit of about 0.001 xcexcm in average roughness is appropriate. Average roughness along the centerline appearing in the present invention is an average roughness measured along the longitudinal direction of a groove by the method specified in JIS B0601 (1994).
A material consisting essentially of an electrically conductive inorganic compound phase that contains a predominant amount of one or more components selected from the group consisting of titanium nitride, titanium carbide, titanium boride, tungsten carbide, zirconium nitride, titanium carbonitride, silicon carbide, and niobium carbide exhibits good electrical conductivity in particular and can be favorably used in the present invention as a material for the electrically conductive ceramic. Alternatively, the electrically conductive inorganic compound phase may be formed of an electrically conductive oxide, such as titanium oxide (e.g., TiO2), tin oxide (SnO2), copper oxide (Cu2O), chromium oxide (Cr2O3), or nickel oxide (NiO). Particularly, an electrical conductive inorganic compound phase that contains a predominant amount of titanium oxide exhibits good electrical conductivity and can enhance the strength of a composite ceramic. Therefore, such is favorably used in the present invention.
Preferably, the above-described conductive ceramic contains an electrically conductive inorganic compound phase in an amount of 50-98% by volume. In this case, the balance other than the electrically conductive compound phase can be a grain boundary phase derived from a sintering aid or, in order to enhance strength, a mixed phase of a grain boundary phase and a ceramic matrix phase formed of any one of alumina ceramic, zirconia ceramic, and silicon nitride ceramic. Since alumina ceramic, zirconia ceramic, and silicon nitride ceramic exhibit excellent wear resistance, selection of such a ceramic as the ceramic matrix can enhance wear resistance and thus can attain compatibility between good electrical conductivity and high mechanical durability.
The dynamic-pressure gap definition surface can be a radial dynamic-pressure gap definition surface located radially distant from the axis of rotation of the bearing. Specifically, the first member is formed into a spindle and is inserted into a reception hole formed in the second member; and the inner surface of the reception hole and the outer circumferential surface of the first member received inside the inner surface serve as radial dynamic-pressure gap definition surfaces, which define a radial dynamic-pressure gap therebetween.
For example, in a dynamic-pressure bearing having a structure shown in FIG. 1, the radial direction is a direction perpendicular to the axis of rotation (extending vertically in FIG. 1) of the spindle. For example, in FIG. 1, the outer circumferential surface of a spindlexe2x80x94which serves as the first member in a fixed condition-and the inner circumferential surface of a bearing memberxe2x80x94which serves as the second member assuming the form of a cylindrical rotation bodyxe2x80x94serve as the radial dynamic-pressure gap definition surfaces. As described below, in the case of a bearing elongated along the axis of rotation, the extent to which sufficient radial dynamic-pressure is generated determines whether or not the axis of rotation is stably supported. Therefore, adjusting surface pores on the radial dynamic-pressure gap definition surfaces as specified in the present invention allows for the generation of sufficient dynamic pressure in the radial dynamic-pressure gap and effectively prevents or restrains adhesion wear, seize-up, or a like problem at the time of starting and stopping.
The dynamic-pressure gap definition surface can be a thrust dynamic-pressure gap definition surface formed at a certain location in the thrust direction relative to the axis of the rotation body. Specifically, the first member is disposed so as to face at least one end face of the second member with respect to the axis of rotation; and the end face of the second member and a face of the first member facing the end face serve as the thrust dynamic-pressure gap definition surfaces, which define a thrust dynamic-pressure gap therebetween.
For example, in the dynamic-pressure bearing having a structure shown in FIG. 1, the thrust direction is the axial direction of the spindle; i.e., a direction along which the axis of rotation extends (the vertical direction in FIG. 1). In FIG. 1, an end face of the bearing memberxe2x80x94which serves as the second member assuming the form of a cylindrical rotation bodyxe2x80x94and a face of a thrust platexe2x80x94which serves as the first member facing the end face of the bearing member with respect to the axis of rotationxe2x80x94serve as the thrust dynamic-pressure gap definition surfaces. The thrust dynamic-pressure gap definition surfaces may be slightly inclined from a plane perpendicular to the axis of rotation. As described below, in the case of a bearing which has a short length along the axis of rotation, the extent to which sufficient radial dynamic-pressure is generated determines whether or not the axis of rotation is stably supported. Therefore, adjusting surface pores on the thrust dynamic-pressure gap definition surfaces as specified in the present invention allows for the generation of sufficient dynamic pressure in the thrust dynamic-pressure gap and effectively prevents or restrains adhesion wear, seize-up, or wringing at the time of starting and stopping.
As shown in FIG. 1, a single bearing can have both a radial dynamic-pressure gap and a thrust dynamic-pressure gap. In this case, the first member (or the second member) as viewed from the standpoint of the radial dynamic-pressure gap and the first member (or the second member) as viewed from the standpoint of the thrust dynamic-pressure gap may be the same member or mutually different members depending on the form of the dynamic-pressure gaps. For example, in the case of FIG. 1, the second member is the bearing member as viewed from the standpoint of either dynamic-pressure gap; and the inner circumferential surface of the bearing member serves as the radial dynamic-pressure gap definition surface, whereas the opposite end faces of the bearing member serve as the thrust dynamic-pressure gap definition surfaces. As for the first member, the spindle is the first member as viewed from the standpoint of the radial dynamic-pressure gap, whereas a pair of thrust plates facing the corresponding opposite end faces of the bearing member is the first member as viewed from the standpoint of the thrust dynamic-pressure gap. The spindle is a nonrotating fixed shaft. Notably, as shown in FIG. 13, a bearing 251 is configured such that a spindle 212 is a rotating member, whereas a cylindrical bearing member 221 is a fixed member.
The dynamic-pressure bearing of the present invention can be configured such that the axial length thereof is longer than the outside diameter of the thrust dynamic-pressure gap definition surface, or the thrust dynamic-pressure gap is not formed such that the inclination of the rotation body during rotation is restricted by dynamic pressure generated in the radial dynamic-pressure gap. This defines, for example, a dynamic-pressure bearing having a long axial length as shown in FIG. 11. When a bearing member 35 serving as a rotation body inclines, the inclination is corrected by the action of pressure generated in a radial dynamic-pressure gap 38. By contrast, the dynamic-pressure bearing can also be configured such that the axial length thereof is shorter than the outside diameter of the thrust dynamic-pressure gap definition surface and such that the inclination of the rotation body during rotation is restricted mainly by dynamic pressure generated in the thrust dynamic-pressure gap. This defines, for example, a dynamic-pressure bearing having a short axial length as shown in FIG. 3. When a bearing member serving as a rotation body inclines, the inclination is corrected by the action of dynamic pressure generated in the thrust dynamic-pressure gaps.
Dynamic-pressure grooves may be formed on the dynamic-pressure gap definition surface. For example, formation of known dynamic-pressure grooves on the outer circumferential surface, which serves as the radial dynamic-pressure gap definition surface, of a rotary spindle can realize far smoother rotation. As shown in FIG. 2(a), a plurality of dynamic-pressure grooves can be formed on the outer circumferential surface of the spindle (on the radial dynamic-pressure gap definition surface) while being arranged at predetermined intervals along the circumferential direction. In the embodiment of FIG. 2(a), linear grooves are arrayed while being inclined at a certain angle with respect to a generatrix of the outer circumferential surface of the spindle. However, dynamic-pressure grooves in any other known form can be used. For example, dynamic-pressure grooves can be used in a so-called herringbone form. Specifically, angle (boomerang-like) grooves are formed on the outer circumferential surface at predetermined intervals along the entire circumference such that tips of the grooves are located on a circumferential reference line. Also, as shown in FIG. 2(b), dynamic-pressure grooves may be formed on the surface of a thrust plate (on the thrust dynamic-pressure gap definition surface). In FIG. 2(b), a plurality of curved grooves are formed on the surface of the thrust plate while being arranged at predetermined intervals in the circumferential direction of the thrust plate, which grooves are curved such that the distance between the center of the thrust plate and a point on each groove reduces gradually toward the inner end of the groove.
The dynamic-pressure bearing of the present invention can be effectively used with, for example, a spindle for rotating a hard disk of a hard disk drive, a spindle for rotating a disk of peripheral equipment, such as a CD-ROM drive, an MO drive, or a DVD drive, for computer use, and a spindle for rotating a polygon mirror of a polygon scanner for use in a laser printer, a copying machine, or a like machine. A bearing used in a rotational drive unit of such precision equipment is subjected to high-speed rotation at a speed of, for example, 8000 rpm or higher (in some cases, even at a speed of 10000-30000 rpm or higher). Application of the present invention to such a bearing enables stable maintenance of fluid dynamic-pressure generated at high level to thereby effectively yield the effect of reducing vibration or the like. Also, the present invention provides a motor having a bearing in which the above-described ceramic dynamic-pressure bearing is used in a rotation output section. Further, the present invention provides a hard disk drive comprising the above-described motor having a bearing and a hard disk rotationally driven by the motor as well as a polygon scanner comprising the above-described motor having a bearing and a polygon mirror rotationally driven by the motor.